So-called "rotating patient" computed tomography (CT) scanners are disclosed in, for example, U.S. Pat. Nos. 4,472,822 to Swift, 4,961,208 to Okada, and 5,036,530 to DiGiovanna et al. In such scanners, the patient is positioned in an upright position between an x-ray source and a bank of x-ray detectors, the source and detectors being fixed relative to one another. The patient is rotated through small incremental angles about a vertical rotation axis as x-rays are passed from the source through the patient to the detectors. For any given focal spot and detector position, a view or projection is obtained which provides data about a given two-dimensional slice of the patient's anatomy within a horizontal scan plane. The patient is then rotated to a new angular position for another view in the same horizontal scan plane. After a desired number of views are obtained in a given horizontal scan plane, the x-ray source and detectors are moved together, relative to the patient, along a vertical translation axis to a new horizontal scan plane to obtain image information about the patient in that plane. A series of such horizontal scans may be taken and the data reconstructed to provide an image of the patient's anatomy.
CT scanners generally employ an array of discrete x-ray detectors arranged on an arc of a circle in diametric opposition to the focal spot. For best resolution, it is necessary to use many small detectors, so that the fan beam can be thought of as divided into many equally spaced tiny rays, each ray being detected by a single detector to provide a single intensity measurement for that ray. The distance between the centers of these detectors is referred to as the "period" of the detector array. The period is equal to the detector width plus the width of the space between adjacent detectors. To ensure that the data are sufficiently sampled, it is known to increase the amount of data taken by positioning the detectors so that they are offset by a predetermined amount from, i.e., not centered relative to, a ray extending from the focal spot and passing through the isocenter of the system. The amount of the offset is typically some fraction of the period of the detector array. This offset of the detectors when they are in one position relative to the patient is manifested as an equal and opposite offset when the detectors are on the opposite side of the patient, thereby doubling the amount of data taken within a single 360.degree. scan. A preferred detector offset is typically 1/4 or 3/4 of the period of the detector array; however, any offset value can be used. Such use of detector offsets is disclosed, for example, in U.S. Pat. No. 4,048,505 to Hounsfield and U.S. Pat. No. 4,176,279 to Schwierz et al.
In a rotating patient scanner, the vertical axis of rotation of the patient will not be exactly parallel to the vertical translation axis along which the x-ray source and detectors move. This is because the detectors in a rotating patient scanner are not fixed with respect to the isocenter of the scanner. In contrast, in CT scanners, in which the patient is held stationary and the x-ray source and detectors rotate about the patient, there is no discernible divergence in the respective rotation and translation axes, because the detectors have a fixed spatial relation to the isocenter of the system. Thus, in the rotating patient scanner, the detector offset will generally vary from scan to scan in proportion to the extent of nonparallelism of the vertical rotation axis of the patient support and the vertical translation axis of the detectors. If not properly handled in the reconstruction algorithm, such a variation in the detector offset from slice to slice may introduce artifacts into the reconstructed image, as the value of the offset for each slice cannot be precisely maintained within a very small error for each scan.
Although the detector offset may be nominally set at a given value, to avoid the introduction of at least some types of image artifacts, it is necessary to determine an actual or true value for the detector offset .delta. for a given set of x-ray intensity data. If the true offset .delta. is known to within a sufficiently small error, the x-ray intensity data from the detectors can be reconstructed using, for example, back-projection algorithms or other known algorithms, to provide an image which is free of artifacts caused by changes in the detector offset over a significant range of offset values.
Although the prior art discusses rotating patient CT scanners, the problem of variation in the detector offset as a function of axial position of the x-ray source and detectors, intrinsic to rotating-patient CT scanners, is not discussed.
It is known to provide various reference structures of known radiation attenuation characteristics within the image field of a CT scanner in order to provide reference data for x-ray density calibration purposes. Such reference structures are typically placed on or in a patient support table, or within a removable patient overlay or support, so that the reconstructed image contains information about both the patient's anatomy and the reference structures. U.S. Pat. Nos. 5,034,969 to Ozaki, 4,870,666 to Lonn et al., 4,651,335 to Kalender et al., 4,233,507 to Volz and 4,782,502 to Schulz all disclose the use of such reference structures for the purpose of calibration or standardization of the image data.
U.S. Pat. No. 5,109,397 to Gordon et al., assigned to the assignee of the present invention, discloses a rotating gantry CT scanner (i.e., a scanner in which the patient is stationary and the x-ray source and detectors rotate about the patient) which includes an x-ray opaque ring surrounding the patient and disposed entirely within the image field. The ring is thus visible in every view of the patient and provides an indication of lateral movement of the source and detectors for each incremental angular position during a scan. This information is used to adjust the image data to compensate for such movement so as to ensure that the data are accurate for image reconstruction using known back-projection algorithms. However, the ring structure of Gordon et al. is used in a rotating gantry scanner. Thus, the problem of variations in the detector offset due to divergent rotational and translational axes of, respectively, the patient and the detectors, a problem inherent only in rotating patient scanners, does not occur. Gordon et al. does not address or provide a solution to this problem.
U.S. Pat. No. 4,860,331 to Williams et al. discloses an image marker device which includes x-ray--opaque reference structures and is attached to the patient's skin in a desired location. The marker device is used to determine the patient's position relative to an external reference frame. However, as the marker device is relatively small, the reference structures are not necessarily present in every image of the patient, and they are not used to measure detector offset so as to adjust the data in the reconstructed image. Furthermore, Williams et al. do not disclose or refer to the problem of errors in the detector offset, and the resulting introduction of image artifacts.
U.S. Pat. No. 4,710,875 to Nakajima et al. discloses an alignment procedure for a digital radiography system in which x-ray--opaque reference points or lines are recorded together with image data in order to provide an indication of any rotation or shift of the imaged features from scan to scan. The reference points or lines are not visible in every image of the patient. Information about the imaged features cannot therefore be used to adjust the reconstruction data to eliminate artifacts caused by changes in the detector offset. Furthermore, Nakajima et al. do not disclose or refer to the problem of errors in the detector offset, and the resulting introduction of image artifacts.
Thus, it would be advantageous to provide a rotating-patient computed tomography scanner which addresses and overcomes these deficiencies, which are not present, and therefore not addressed, in prior art rotating gantry scanners.